I would mention the other school, I don't believe they can rescind an acceptance unless you don't send in the deposit by the deadline, you lied on your application, etc... If she never stated to the other school that they were her first choice, all is fair in love and war. Sending in a letter of reconsideration can not hurt...However, it should be very clear in the letter why your friend feels he/she would be a good candidate and what specifically it is about the school that makes your friend want to attend. I hope they have already visited, and can refer to their personal impressions of the school. In other words, it should be one hell of an outstanding letter that your friend is able to sell their self out side of the LSAT score, gpa, or anything else she might have already stated in his/her personal statement. Your friend also needs to be prepared to get rejected from them again, if they are willing to put their self through that again, go for it! Who knows maybe they will defer your friend if he/she is willing to attend next year...Stranger things have happened.